This invention relates to offshore jacket tower structures for use in deep water of 750 feet and more such as for example several thousand feet or more. Fixed jacket tower structures contemplate a tapering tower including three or more legs interconnected by various types of bracing utilizing structural steel sections and cylindrical members. Such tower structures have heretofore been constructed and used in relatively shallow water depths. Construction and use of prior proposed jacket tower structures for deep water becomes very expensive and the stresses involved in floating, transporting and erecting such prior proposed deep water towers require extensive use of truss type bracing because of the great weight of such a steel structure and also because of the forces imparted to the structure during transport and floating by wave and weather conditions.
Prior proposed jacket tower structures have included towers utilizing three legs such as shown in U.S. Pat. No. 2,586,966 in which three cylindrical legs are arranged in triangular section and interconnected by bracing of apparently structural steel sections. Other prior proposed tower structures include four jacket legs such as shown in U.S. Pat. No. Re. 28,614 in which the jacket tower comprises four legs and is supported for transport and flotation on a transport and launch apparatus comprising a pair of buoyant members extending the length of the tower and associated with two of the legs. The buoyant members are utilized in transferring the jacket tower from a horizontal floating position to a vertical upright position on the sea bed after which the buoyant members are separated from the tower.
A jacket tower assembled at sea by joining two or more jacket sections is shown in U.S. Pat. No. 3,859,806 in which jacket legs are provided with compartments provided with sea cocks, one acting as a flooding valve and the other as an air escape means, the sea cocks being operated by suitable power means. A crane on a barge is used to control the upper end thereof as the legs are flooded as the tower changes its position from a horizontal to a vertical position. This patent also shows means for joining two floating sections by pulling them together by winch means at least one leg being at the water surface and the other legs submerged.
U.S. Pat. No. 3,641,774 shows another method for assembling and completing fabrication of a multi-jacket section structure in which the jacket sections are guided into alignment, clamped together and then welded and in which a coffer dam device is utilized during the joining operation. Two parallel legs are at the water surface and a third parallel leg lies above the water surface.
U.S. Pat. No. 2,857,744 shows a support structure having three parallel legs interconnected by bracing adapted to be flooded during tilting of the structure to a vertical position from a horizontal position in which two legs are at the water surface and a third leg is above the surface.
While many of the prior proposed jacket tower structures which comprised a plurality of jacket sections were not designed for use in deep water the extension of such structures for use in deep water without modifying their design becomes extremely costly. The size and type of the jacket structural members and thickness of metal section required for use in deep water installations increases the costs of such structures so that they become prohibitive cost wise as well as imposing great difficulties in handling the several jacket sections of such large and long dimension.